Low power, short range remote control has had historical usages with, for example, garage door openers. Such remote controls often include simple, low cost digital transmitter/receiver pairs operating in the unlicensed industrial, scientific and medical (“ISM”) radio bands. An ISM radio band frequently used in the prior art was in the 270-470 MHz range.
Garage door openers and other types of simple, digital remote control systems have used saw-based oscillator transmitters having simple on/off key encoding. The transmitters typically operated in the 270 MHz to 470 MHz frequency range and had an operable distance range of approximately 50 to 100 feet at permissible power levels.
It has been desirable to keep the power requirements of such devices low, particularly with respect to the transmitter, and to use simple, discrete circuitry to keep down costs. For example, the circuitry of a traditional remote control may be comprised of a few transistors and passive components. The receivers traditionally were of an inexpensive super-regenerative design with wide bandwidths, e.g. 2-3 MHz. However, because of their wide bandwidths, the signal-to-noise ratio (SNR) of such receivers was fairly low, placing a limit on their effective range.
It will therefore be appreciated that remote controls of the prior art, while being inexpensive, suffered from the problems of having a very short effective range. Most methods for extending operation distance range of such super-regenerative designs were focused on the transmitter. For example, the power of the transmitter could be increased or a higher-quality antenna could be used. However, the battery operation and typically small size of remote control transmitters often work against such solutions as well as power restrictions in operating in the ISM radio bands. For example, remote controls for automobile door locks are often the size of key fobs and have very small batteries and limited room for an antenna. There is therefore a move in the industry to upgrade the prior art super-regenerative receivers to a more sensitive superheterodyne receiver design.
Superheterodyne receivers, while generally superior to super-regenerative receivers, require relatively expensive crystals and integrated circuits. Despite the additional costs, superheterodyne receivers tend to be favored because they have frequency resolutions which are an order of magnitude better than those of super-regenerative receivers. For example, where a super-regenerative receiver might have a pre-demodulation bandwidth of 3 MHz, a superheterodyne receiver might have a pre-demodulation bandwidth of 300 KHz. The lower pre-demodulation bandwidth of the superheterodyne receiver greatly increases the SNR of the received signal, increasing the effective range of the remote control or remote monitoring device.
The greater effective range afforded the superheterodyne receiver comes, as noted above, at the cost of relatively more expensive circuitry. For example, both the transmitter and the receiver of the remote control require crystals and an integrated circuit. Nonetheless, there is a continuing desire to increase the effective range of remote control and/or monitoring systems such as for automatic meter reading (AMR), remote keyless entry (RKE) for automobiles, and home automation.
One trend is to operate in a higher ISM radio band in order to increase effective range. For example, there is continuing investigation to use higher frequency ISM bands, e.g. 800-900 MHz. This is because operating at higher frequencies allows for a shorter, higher quality antenna to fit within the size constraints of the transmitter. A higher quality antenna will typically result in an increased effective range.
The user of higher frequencies in superheterodyne receivers present new problems, one of which is crystal resonance frequency “error”, e.g. any factor (such as tolerance, aging, drift, temperature effects, etc) which causes a crystal to have a resonant frequency other than that which is desired. As used herein, “error” will mean any deviation from the desired resonance frequency of the crystal. As the crystal frequency drifts or otherwise moves from a desired frequency to create a crystal frequency error, the PLL which uses it as a reference will also be subject to a proportional frequency error. For example, when superheterodyne receivers are operated in the 270-470 MHz range, a crystal induced local oscillator frequency error of up to about 50 KHz is acceptable. However, with higher frequency operation and/or narrower intermediate frequency (“IF”) bandwidth utilized to further increase range, the frequency tolerance and temperature coefficients of the crystals must be much better, often requiring expensive compensated crystals at both the receiver and the transmitter.
This problem will be explained in greater detail with references to FIGS. 1A, 1B and 1C. In FIG. 1A, an example superheterodyne receiver 10 includes an antenna 12 and a low noise RF amplifier 14 for amplifying the signal detected on antenna 12. A local oscillator 16, which includes the crystal, has an output which is mixed with the output of the RF amplifier 14 in a mixer 18. The output of the mixer 18 is filtered in filter 20 and then amplified in an IF amplifier 22. The output of the IF amplifier 22 is demodulated in demodulator 24 and the resultant audio amplifier 26 creates the output for the receiver 10.
As noted above, at higher frequency operation the absolute error of the local oscillator frequency becomes proportionally larger compared to receivers operating at lower frequencies. As seen in FIG. 1B, the circuitry of FIG. 1A operates well when the IF signal is fully within the IF filter frequency response of the receiver. However, with crystal error, the intermediate frequency of the signal can shift such that the IF signal is no longer within or fully within the IF filter frequency response. Furthermore, the transmitter and receiver crystal-based errors are cumulative. This “major frequency error” problem is illustrated in FIG. 1C.
A partial solution to crystal error is to use expensive, high quality compensated and/or temperature characterized crystals. An example of this is to use an external temperature compensated crystal oscillator (TCXO). However, this solution is undesirable to manufacturers who wish to keep the manufacturing costs of the devices as low a possible. It would therefore be desirable to find a solution to the crystal error problem which allows the use of relatively low cost crystals in both the transmitter and receiver of remote control and/or monitoring devices.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.